Superheterodyne detector



Nov. 7, 1939. K. w, Am/ls ET AL 2,178,995

' SUPERHETERODYNE DETECTOR Filed D60. 24, 1936 A W5 K /9 20 *N 3 Q *Nif/all L 4MM f4) nl; l AMEN!!! f'iillllllI* Lfo .I/a l@ I/7 f Wa 3 37 H NiMH-- 44 lNvENToRs KENNETH W. JARVIS AND Patented Nov. 7, 1939 UNETED STATES SUPERHETERODYNE DETECTOR Kenneth W. Jarvis, Norwalk, and Russell M. Blair, Westport, Conn., assignors to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application December 24, 1936, Serial No. 117,658

4 Claims.

The present invention relates to signalling systems and, in particular, to circuit arrangements adapted for use in connection with the operation of electron and vacuum tubes of the general types described in our U. S. Patent No. 1,903,569, issued on April 11, 1933, on which a reissue application Serial No. 16,108 has been led on April 8, 1935. Still further and more particularly this invention relates to the use oi primary and secondary electron streams and associated tube and circuit elements and operating potentials for producing systems having a high degree of detecting eiliciency.

An object of our invention is toprovide in a single tube structure with coacting tube and circuit elements, a dernodulator of modulated high frequency and an amplifier of the modulation frequency.

Another object of our invention is to provide a demodulating system wherein the modulation frequency is ampliiied while the modulated frequency is suppressed.

A still further object is to provide for the simultaneous combination of eiects produced by nonlinear response of two separate frequencies, while preventing mutual interaction between their sources.

Another object of our invention is to provide highly efcient detector, whose detecting erliciency is substantially unaffected by its output circuit.

A still further object of our invention is to combine t-he eects of an ordinary grid-potentialanode current characteristic of a triode with the secondary emission characteristic of an electron multiplier to produce a new and more efficient demodulator.

A further object of our invention is to provide a more sensitive detector by utilizing the threshold effect of secondary emission. l

Still other and further objects will be apparent in the following description and in the appended claims.

Three figures are attached for better description. Fig. 1 shows a simple circuit arrangement utilizing the tube and circuit of our invention:

Fig. Z discloses curves where current is plotted against time in order to show the relationship between control grid potential and several of the currents in the tube; and

Fig. 3 discloses a modification of the circuit of Fig. 1 where the circuit structure is particularly designed for the simultaneous detecting or mixing of two frequencies.

In Fig. 1, the reference character I indicates (Cl. Z50-20) the electron tube referred to above. The numbers 2 and 3 refer to the input terminals where the modulated high frequency potential is impressed. The transformer d is tuned substantially to the incoming frequency by the condenser 5. The high potential end of the transformer d is connected to the control grid 6 of the electron tube I, while the low potential end of the transformer d is connected through the polarizing potential l to the filament electron emitter 8, also an element in tube I. A collector plate 9 is provided, and connected back to the emitter 8 through the source of potential Ill. Another element in the tube I is the output plate II, which is connected through the inductance l2, to the low frequency transformer I3, and the source of potential Ill back to the emitter 8. There is also provided the shielding element l5, which is similarly connected back to the emitter 8 through the source of potential I6. The collector plate 9 is treated or otherwise made to be productive of high secondary emission of electrons from impact of a primary stream of electrons. Such a primary stream isindicated at l l, and flows from the emitter 6 to the collector plate 9. Due to the impact of the impinging electrons and the properties of the electrode 9, a greater number of charges are emitted than are received in View of the potentials applied. These secondary charges flow to the output plate II as indicated by the reference character lll. The shield l5, its position in relation to the other electrodes, and its applied potential determine the paths of the charge streams il and I8. A more detailed description of the purpose and action of this screening element I5 will be found in the Patent No. 1,903,569 referred to above.

Itis assumed that proper operating potentials are applied to all the elements of the tube, including the source of potential l used for heating the emitter t to render it electron emissive. The potential Ill is made high enough so that secondary emission saturation from the collector plate 9 throughout the operating range desired is obtained. Fig. 2 is a plot of voltage between the control grid 5 and the primary emitter S, with respect to the two emission currents ll' and i6 previously mentioned. The curve indicated as In in Fig. 2 shows the magnitude of the charge stream flowing between the emitter 8 and the collector plate 9 as the potential on the grid 6 is varied. This curve is quite similar to such a curve for the normal and well known three element electron tube. If an operating point is chosen, such as shown by the dashed line 2|,

Where the value of the polarizing potential E7 is shown, it may be observed that the characteristic Iri is very definitely curved. As is well known in the art, if a modulated signal be superimposed on the voltage E7, so that the instantaneous potential on the grid 6 is varying symmetrically about the potential E7, a non-symmetrical current variation is produced. The effect of this non-symmetrical current relationship is to produce, in addition to the applied high frequency, components corresponding to currents of the fundamental, and multiple, modulation frequencies. As it is the fundamental modulation frequency component that is generally desired in detection, it is obvious that this gives a means of detection. It is also well known in the art, that the greater the rate of change in curvature,

dz dE2G-r the more sensitive will be the detector. It is to obtain this higher degree of curvature that the tube and associated circuits of our invention function.

By our invention, we can obtain a greater rate of change of curvature as shown by the characteristic Ira. This is obtained in the following fashion: Assume zero potential on the collector plate 9, while the output plate ii is maintained at constant potential above the collector plate 9. Under these conditions no current il' will oW, nor will there be any current ivi as there is no primary emission to dislodge the secondary charges. Now let the voltage il! be gradually increased. The current i'i will be-- to rise as more and more electrons are attra to the surface of the collector plate S. At first, however, these electrons have barey enough velocity to reach the plate, and so are incapable oi dislodging any secondary emission. As the ceilector plate d potential lli is raised, the primary emission gains in velocity and finally a few primary electrons acquire sufficient kinetic energy to dislodge secondary charges. These are drawn over to the output pate and appear in Fig. 2 as the intersection of Ils and theabscissa. As the potential on the collector plate is still further increased more primary eiectrons reach the collector plate, and with a much higher velocity. This twofold effect, of greater numbers and greater velocities, serves to very rapidly increase the secondary emission from the coilector plate Q, all of which is drawn over to the collector plate H. As a result of this eifect, the current l5 increasesl with extreme rapidity as the collector plate potential is raised and is shown by the curve Iig in Fig. 2. Using a xed collector plate potential, exactly the saine effect is produced by raising the grid 6 from a negative value which produces current cutoff toward a positive value as shown in Fig. 2. As a result, a very sharp break in the curve results, and entre. Le sensitivity is obtained, since the curvature of Iig is due to product of In multiplied by the variable ratio of secondary to primary emissions.

As the potential is further increased as described, a point is reached at which very little further increase in ratio of secondary emission to primary emission is obtained. The reason for this action is somewhat obscure, but a possible explanation has been given in a copending application Serial No. 15,2712, filed April if), i935, and entitled Electron tube circuits. After this limit has been reached, the current Ils is a constant multiple of In and equals the secondary emission ratio. As this may be at a point in the characteristic curve which is still curved, detection can take place as before described, except with the utilization of the secondary emission ratio to increase the sensitivity.

Thus there are two operating conditions for satisfactory detection. First, relying on the normal curvatures as described. Second, utilizing the sharp curvature produced at the threshold of secondary emission. With either of these conditions an added feature is possible in our invention. Reference is made to the choke coil inductance l2 in Fig. l. This inductance is chosen to oppose the iiow of currents at the frequencies impressed on the input terminals i. and 3, while having little or no effect on the currents of niodulation frequency for which the transformer I3 is effective. If such an impedance were included as shown in the plate circuit of a normal detect ing system, the detecting efficiency would be greatly reduced due to the straightening out of the curve due to the Well known dynamic action. In the system of our invention, the high frequency variations in current may be completed through the collector plate 9 and the battery l!) back to the emitter 8. As a result, only the modulation frequency variations need to iiow from the collector plate 9 to the output plate Il and through the other circuit elements IZ, i3, iii and lil. Therefore, any desired means, such as the choke i2 or by-pass condenser may be used in the output circuit to prevent the high frequency input currents from being amplified. and passed on to the modulation frequency channel, with a total disregard of their dynamic effects on the detection sensitivity.

It will be appreciated that this advantage is secured because the impedance of the path between the eiectrode li and emitter no effect upon the dynamic characteristics of the path between the emitter 8 and the electrode 9. Thus complete iiltering may be obtained with no loss in performance, often an important consideration.

In Fig. 8 is shown a composite or dual detector wherein two frequencies may be heterodyned to produce sum or difference frequencies with a ccnsiderable degree of independence. The tube is indicated at 22, and contains an emitter 23, collector plate M output plate 25. a shield element 2G, and tivo grids indicated at 2l and 28 for controlling the electron streams 29 and 30 respectively. The emitter is rendered thermionic emissive by heating by means of the source of potential 3i. The input circuit to the first grid 2l is shown by the input terminals and which feeds the tuned circuit indicated at 3d. The grid circuit of grid 2i is closed through the tuned circuit 34 and the polarizing potential 35 back to the emitter 23.

rIhe terminals 35 and 3l serve as input points for the signal controlling the grid 23. The grid 25 circuit is closed through the tuned circuit 38 and the polarizing battery 33 back to the collector plate 2li. t is important to note that this grid circuit returns to the collector plate 2li and not to the emitter The equivalent three element tube wherein the grid 2d exercises control comprises the coilector plate 2li acting as the emitter, the control grid 2S and the output plate 25. It is true that the emission from the collector plate 24 is determined by the secondary emission ratio of the material on this plate ifi, and the primary emission striking this collector plate, but otherwise the elements 2d, 2d and 2t are a completely independent acting thermionic device analogous to the conventional three element thermionic tube. The grid 28 is biased negatively With reference to the emitter 2 by the source of potential 39.

The collector plate 2t, energized by the source of potential 139, serves as an anode analogous to a conventional three element tube, the only unusual feature being that this collector plate has no impedance between itself and the low potential input, as would be necessary in any other system where the effects in this first tube are to be transferred to a second tube. As before explained in the above referred to Patent No. 1,903,569, the relay action of the secondary emissionat the collector plate 2li is the sole means of combining these equivalent independent tubes.

The output plate 25 is connected through the output circuit i3 and the source of potential il to the emitter 23. It should be noted that if the current 33 is greater than the current 29, which is generally the case, current ilows through the source of potential 4U in the reverse direction and thus absorbs rather than delivers power, This power is furnished by the source of potential 4 The shield 26 is maintained at a desired potential by the source of potential 42.

One source of common coupling is possible even in these equivalent tubes coupled only by the electron streams at the collector plate 2li.v This occurs as follows. When controlled by a variable voltage on the grid 2l', the current 29, and consequently the current 3i), will vary likewise. There is a capacity effect between the variable current 30 and the grid 28. As a result, the variable effect produced by the potentials on the grid 27, may be found on the grid 28, and so be effective in the circuits connected with the input circuit 38. In order to avoid this effect, it is desirable to place the potential having the most stable frequency on the grid 28, rather than on the grid 21. As an example, suppose the circuit of Fig. 3 be used as a rst detector in a superheterodyne. The local oscillator should be used to excite the grid 2l, and the distant radio signal should be used to excite the grid 23. This is because the local oscillator can hardly effect the frequency of the distant and powerful station, and the re-l verse control is prevented by the one way action of the secondary emission relay at the collector plate 24.

For proper combination, it is desirable that both the equivalent tube 23, 2l, 24, and the equivalent tube 2A, 28, 25 be operated on a nonlinear characteristic as before noted. When this is done, the tvlo applied frequencies will give sum and difference terms as the frequency of the output.

In case the two applied frequencies are high and close together, a low or intermediate frequency is obtained in the output, as well as much higher frequencies. The lower frequency is generally-desired, and this operation is usually known as heterodyne detection. If one frequency applied is high, and the other low, the ,sum and difference frequencies are both usually desired as they are the so-called sidebands of a modulated transmission. Such a process is known as modulation, and is well and easily accomplished by the teachings of our invention.

In the operation of the system of Fig. 3, it is usually desirable to have the `tuned circuits 34, 3B and 43 resonant at the desired frequencies and, of course, other networks may be used in place of the illustrated simple ones. If it is desired, a

series inductance can be connected between the electrode 25 and the tuned circuit i3 for suppression of certain frequency components as point-ed out above in the description of Fig. l.

Having described our invention, what we claim 1s:

1. A superheterodyne detector comprising means for supplying carrier wave energy, means for supplying signalling energy, means for pro-y ducing a stream of primary electrons, means for controlling the magnitude of the produced stream of electrons in accordance with the carrier wave energy, means for producing a stream of secondary electrons in accordance with the number of the controlled primary electrons which exceed a predetermined velocity, an electrode for collecting secondary electrons, and grid control means energized by the signalling energy and in- `terposed only in the produced stream of secondary electrons for4 controlling the number of electrons which reach the collector electrode.

2. A superheterodyne detector comprising means for supplying carrier wave energy, means for supplying signalling energy, means for producing a stream of primary electrons, means for controlling the magnitude of the produced stream of'electrons in accordance with the carrier wave energy, means for producing a stream of secondary electrons in accordance with the number of the controlled primary electrons which exceed a predetermined velocity, means for-'collecting secondary electrons, and grid control means energized by the signalling energy and interposed only in the produced stream of secondary electrons for controlling the numberof' electrons reaching the collecting means.

3. A superheterodyne detector comprising means for supplying carrier wave energy, means for supplying signalling energy, means for producing a stream of primary electrons, means for controlling the magnitude of the produced stream of electrons in accordance with the carrier wave energy, means for producing a stream of secondary electrons in accordance with the number of the controlled primary electrons which exceed a predetermined velocity, means for collecting secondary electrons, grid control means energized by the signalling energy and interposed only in' the produced stream of secondary electrons` for controllingthe number of electrons reaching the collecting means, and means for transferring the charges of the collected electrons to an output circuit.

4. A superheterodyne detector comprising means for supplying carrier wave energy, means for supplying signalling energy, means for producing a stream of primary electrons, means for controllingy the magnitude of the produced stream of electrons in accordance with the carrier wave energy, means for producing' a stream of secondary electrons in accordance with the number of the controlled primary electrons which exceed a predetermined Velocity, means for collecting secondary electrons, grid control means energized by the signalling energy and interposed only in the produced stream of secondary electrons for controlling the number of electrons reaching the collecting means, means for transferring the charges of the collected electrons to an output circuit, and means for subsequently suppressing predetermined energy components of the transferred charges.

KENNETH W. JARVIS. RUSSELL M. BLAJR. 

